The Bridgestone tires are a different size (lower profile) than the
Michelins, which have a slightly taller sidewall.  Tread patterns differ
from manufacturer to manufacturer.

Why would you put a different size tire on the front?  The Michelins
are NOT the same size as your old Bridgestones -- 50R18 instead of
40R18.  Good thing you don't have all wheel drive, or you would be in
a bit of trouble.

Also, your speedometer/odometer readings will also be off by a
considerable amount with this change.  If you change the
width/diameter ration, you have to also change the diameter to keep
the same tire circumpherence.

Unless your 50R18 is just a typo.  :-)

First of all, Michelins are much better tires than the Bridgestones
you replaced. Like others said, these are taller profile (50) tires
than the typical 40 you replaced.

According to the Michelin website, the "standard" is the 225/40R18
(88Y front / 95Y rear). Your specific model year may allow the 225/50
if the manual says so; otherwise you have the wrong tires on there.
I'd use only the specified geometry but others may differ.

Who put those tires on there?

http://www.michelinman.com/action/tires/find-by-vehicle/?make=Lexus&model=IS350&
year=2007&option=RWD+18+Inch+Tire+Option